Specific Aim 1: To study the diagnostic accuracy of stress CT perfusion As mentioned, CT scanners can acquire high quality images of the coronary arteries. CT coronary angiography has high diagnostic accuracy in the great majority of patients but may not have sufficient resolution or image quality to assess the patency of coronary stents and some heavily calcified vessels. Using CT to image stress myocardial perfusion could help solve the diagnostic uncertainty in those special subsets of patients. In addition, while high resolution overall, the resolution of a CT scan for grading the severity of a coronary stenosis in a 3 mm artery is still marginal. This is another area where CT stress perfusion could provide the physiologic assessments needed to help determine whether an anatomic stenosis is severe enough to have caused ischemia or warrant intervention. To address these issues, we have studied 58 patients as part of the multicenter Core320 study which compares CT coronary angiography/ stress CT perfusion with quantitative invasive coronary angiography/SPECT perfusion. We were the leading US recruitment site for this study which recently closed enrollment. Results are pending. In addition, we are leading an ancillary study that includes comparison of CT perfusion with stress MRI perfusion. That ancillary study has also completed enrollment and is under analysis. Specific Aim 2: To develop low dose cardiac CT imaging protocols that retain high diagnostic accuracy. Another pressing research direction has been developing low radiation dose CT scans of the heart. The public and academic medical communities have both realized that radiation exposure from diagnostic imaging is increasing at a rapid pace and may be associated with some health risks. Thus, finding methods for reducing radiation exposure in CT scans of the heart is a significant priority. To the best of our knowledge, the NHLBI cardiac CT program has the lowest average radiation exposure of any site using the 320 detector CT scanner. We are leading an assessing even lower dose CT imaging protocols. Preliminary results have been presented at National meetings and are in the process of being written.